Certain Angiotensin Converting Enzyme (ACE) inhibitors are unstable in that, they are highly susceptible to decomposition by cyclization hydrolysis and oxidative attack. It is believed that one or more of these types of decomposition causes the discoloration which results when these compounds are formulated into pharmaceutical dosage forms. Antihypertensive compounds, such as quinapril and enalapril, undergo autocyclization to the undesirable diketopiperazine. In addition, they may form diacids via hydrolysis or may undergo oxidation leading to discoloration. It is their conversion to these sometimes unwanted substances which can result in lessened drug effectiveness in compositions containing this type of drug.